Most cars have a cable on the driver's side that, when pulled, will release the latch on the front hood of the car. When the bonnet won't open, it can mean either the cable leading to the latch is broken or that the actual latch assembly is not working. There is a way to open the bonnet when the latch itself is broken.
Determine the cause of the bonnet not opening. If the release handle for the bonnet doesn't retract back into position after you pull it, it's likely that the cable leading to the latch is broken. If the release handle retracts as usual and you can feel it trying to release the latch, it may be the latch itself that is broken. It is more common for the latch to be broken or jammed than for the cable to be broken.
Rule out the bonnet being jammed shut before you proceed as if the latch is broken. Have another person stand in front of the car and pull up on the bonnet while you stay inside the car and pull on the bonnet release cable. This may help unjam the bonnet if that is the problem. If this still doesn't help the bonnet lift up, you can proceed as if the latch unit is broken.
Shine a flashlight through the front grill of the car to locate the latch unit. Once you can see it, use a screwdriver to go through the front grill and manipulate the screwdriver in different ways, trying to open the latch. The latch is spring loaded, so it may take pressure to open the latch manually. Be careful to stay away from the condenser for the air conditioning unit--the screwdriver could puncture a hole in it if you don't keep it only on the latch assembly.
It may take a lot of time and patience to find the right manipulation to open the latch unit as well as the right amount of pressure to open it without puncturing the air-conditioning condenser. Proceed cautiously, as you can always try greater amounts of pressure.